1. Field of the Invention
The present invention relates to a photoelectric converting apparatus having a plurality of photosensors, and a plurality of transistors of the insulative gate type which select the signals from the photosensors and output them to a signal line and, more particularly, to a photoelectric converting apparatus for allowing the signal outputs from the photosensors to be read out without reducing the level of those signal outputs.
2. Relates Background Art
FIG. 1A is a schematic circuit diagram of a scan switching section in a conventional photoelectric converting apparatus. FIG. 1B is a schematic plan view showing a constitution of the scan switching section.
In FIG. 1A, signals from photosensor cells S.sub.l to S.sub.n are accumulated in capacitors C.sub.l to C.sub.n, each having a capacitance C.sub.t. Thereafter, these signals are sequentially transmitted through transistors Q.sub.l to Q.sub.n and are read out to a signal line 1 and are serially output through an output amplifier 3. The ON/OFF operations of the transistors Q.sub.l to Q.sub.n are controlled by pulses .phi.h.sub.l to .phi.h.sub.n which are generated from a scan circuit 2.
The transistors Q.sub.l to Q.sub.n are MOS type transistors having a pattern as shown in FIG. 1B. Each of these transistors is an n channel MOS transistor comprising: a drain region 4 and a source region 5 each of which consists of an n.sup.+ semiconductor; and a gate electrode 6 provided through an oxide film. The drain regions 4 of the respective cells are connected to the capacitors C.sub.l to C.sub.n. On the other hand, the source regions 5 are commonly connected to the signal line 1. Therefore, for example, when the positive voltage pulse .phi.h.sub.l is applied from the scan circuit 2 to the gate electrode 6 of the transistor Q.sub.l, the transistor Q.sub.l is made conductive and transfers the signal (signal charges Q) accumulated in the capacitor C.sub.l to the signal line 1. The other transistors are also similar to the transistor Q.sub.l.
In this case, since the signal line 1 itself has a capacitance C.sub.h, when the signal charges Q accumulated in the capacitor having the capacitance C.sub.t are transferred to the signal line 1, a voltage V.sub.h appearing on the signal line 1 is set to Q/(C.sub.t +C.sub.h).
However, in the foregoing conventional photoelectric converting apparatus, the drain region 5 of each transistor is connected to the signal line 1. Therefore, the ratio of the capacitances of the drain regions 5 to the stray capacitance C.sub.h of the signal line 1 is large, so that there is a problem such that the voltage V.sub.h appearing on the signal line 1 decreases and the S/N ratio of the signal is deteriorated. This problem is typical in the case of a high resolution sensor in which a number of photosensors are arranged.
For example, in FIG. 1B, the capacitance of the drain region 5 of the transistor Q.sub.l connected to the signal line 1 is calculated. First, when the capacitance per unit area is set to 5.times.10.sup.-5 pF/.mu.m.sup.2, the capacitance of the bottom surface of the drain region 5 is about 0.005 pF. On the other hand, when the capacitance per unit area is 1.5 pF/.mu.m.sup.2, the capacitance of the side surface is about 0.005 pF. By adding these values, the capacitance of the drain region 5 is about 0.01 pF.
Therefore, assuming that 100 (n=100) sensors are arranged, for example in a row, the capacitance which is applied to the signal line 1 by the transistors Q.sub.l to Q.sub.n is set to about 1 pF. Since each capacitance of the capacitors C.sub.l to C.sub.n to accumulate the output signals is a few pF, the value of about 1 pF is large enough to cause the deterioration of the output signals. As explained above, the conventional apparatus has a problem in deterioration of the outputs in the case where, in particular, a number of sensors are arranged disposed in the photoelectric converting apparatus.